Better ocean turbulence models to improve climate predictions

"Like storms in the atmosphere, these eddies help to distribute energy, warmth, salinity and other things around the ocean," researcher Baylor Fox-Kemper said.

Researchers have found a way to more accurately and efficiently model mesoscale eddies across the Earth's oceans. Photo by Fox-Kemper Lab/Brown University

Feb. 27 (UPI) -- By more accurately modeling the dissipation of turbulence across the planet's oceans, scientists may be able to offer more precise climate change predictions.

In a new study, published this week in the journal Physical Review Letters, scientists present a new method for simulating the behavior of mesoscale eddies, ocean swirls measuring anywhere from a couple dozen to a few hundred miles wide.

Mesoscale eddies are smaller, circular currents that spin off of boundary currents like the Gulf Stream. Their dynamics are altered as they come into contact with pockets of water with different densities and temperatures.

"You can think of these as the weather of the ocean," Baylor Fox-Kemper, an associate professor and researcher at Brown University, said in a news release. "Like storms in the atmosphere, these eddies help to distribute energy, warmth, salinity and other things around the ocean. So understanding how they dissipate their energy gives us a more accurate picture of ocean circulation."

Current simulation see the dissipating energy of eddies translated onto smaller and smaller scales. As the large eddy dies, it degrades into smaller and smaller swirls. But this dynamic is only applicable to small, three-dimensional eddies, not mesoscale eddies.

Mesoscale eddies encompass wide swaths of the ocean surface, and yet the ocean stretches only a few miles deep, making mesoscale eddies essential two-dimensional.

"And we know that dissipation works differently in two dimensions than it does in three," Fox-Kemper said.

Mesoscale eddies follow a totally different logic, combining with other eddies to grow in size over time.

"You can see it if you drag your finger very gently across a soap bubble," Fox-Kemper said. "You leave behind this swirly streak that gets bigger and bigger over time. Mesoscale eddies in the global ocean work the same way."

Until now, scientists have struggled to develop algorithms that accurately describe this upscaling tendency. To solve this problem, Brown researchers looked to a high-resolution ocean model with an impressive track record of matching the direct satellite observations of the ocean dynamics.

Scientists were particularly interested in how the model accounted mathematically for eddy dissipation.

When researchers looked at movement of energy during eddy degradation across five years of climate model simulations, they found the dissipation followed a lognormal distribution. A lognormal distribution features a probability distribution in which the average is heavily weighted at the tail end.

"There's the old joke that if you have 10 regular people in a room and Bill Gates walks in, everybody gets a billion dollars richer on average -- that's a lognormal distribution," Fox-Kemper said. "What it tells us in terms of turbulence is that 90 percent of the dissipation takes place in 10 percent of the ocean."

The model used in the study is high-resolution, but scientists believe the insights it provided will help them design coarser-grained simulations capable to predicting the dissipation of eddies across much larger scales.

Coarser, larger-scaled models are essential to modeling global climate change across long timescales. High-resolution models require too much time and computational resources for large-scale predictions.

"If you want to simulate hundreds or thousands or years, or if you want something you can incorporate within a climate model that combines ocean and atmospheric dynamics, you need a coarser-grained model or it's just computationally intractable," Fox-Kemper said. "If we understand the statistics of how mesoscale eddies dissipate, we might be able to bake those into our coarser-grained models. In other words, we can capture the effects of mesoscale eddies without actually simulating them directly."